The FCC process has in recent times become the major system by which crude oil is converted into gasoline and other hydrocarbon products. Basically, the FCC process includes contacting a hot particulate catalyst with a hydrocarbon feedstock in a riser reactor to crack the hydrocarbon feedstock, thereby producing cracked products and spent coked catalyst. The coked catalyst is separated from the cracked products, stripped and then regenerated by burning the coke from the coked catalyst in a regenerator. The catalyst is heated during the regeneration by the burning of the coke. The hot catalyst is then recycled to the riser reactor for additional cracking. Exemplary of these FCC processes are Haddad et al., U.S. Pat. No. 4,404,095, Lane, U.S. Pat. No. 4,764,268, Quinn et al., U.S. Pat. No. 5,087,427, Forgac et al., U.S. Pat. No. 5,043,058 and Schwartz et al., U.S. Pat. No. 5,089,235.
For the most part, the existing FCC systems were built to provide relatively long riser reactors in order to increase riser cracking and decrease subsequent dense bed cracking in the disengaging or stripping vessel. A typical conventional riser reactor provides a residence time of less than 10 seconds, and usually on the order of 1-5 seconds. See generally, Krambeck et al., U.S. Pat. No. 4,978,440.
Recently, the decrease in available lighter feedstocks has resulted in the need to employ heavier hydrocarbons as the FCC feedstock. Further, catalysts having improved activity have also become available. Also the need to make lighter products can also make it desirable to operate at substantially higher reactor temperatures than are usually employed in conventional FCC operations. It has been found that it is beneficial to employ shorter residence times in FCC processes in which heavier hydrocarbons are employed as the feedstock and/or where high activity catalyst and/or higher (greater than 1000.degree. F.) reactor temperatures are employed. Thus there is currently a need in the industry to provide a system for cracking these heavier feeds and/or employing high activity catalysts and/or higher reactor temperatures and which can be adapted from current existing systems in FCC refineries.